The state of the art relating to the clean-up of polluted water includes pump & treat (P&T) systems, in which a submerged pump or a pump installed on the site, feeds polluted water from a well to a water treatment plant installed on-site where the water is depolluted below clean-up levels. One result of the pumping is the creation of a cone of depression of the water level towards which the polluted water flows together into the well by gravity. The cleaned water is then reinjected into the aquifer by another well. This method, however, becomes a hazard to nearby buildings because the cone of depression of the groundwater level causes unequal rates of compaction of the soil after dewatering of the area above the cone during a long period of P&T application. The remediation activities frequently take years since only that part of the water is cleaned which corresponds to the natural throughput of the well. The on-site treatment of the water in special waste water treatment plants also is expensive. Further, P&T systems are technically not feasible in wells having only a small throughput, in shallow aquifers and in fizzurized rock.
U.S. Pat. No. 4,497,370 discloses an in-situ method for the recovery of liquid hydrocarbons by flotation in specially designed in-well recovery units (chambers) which are designed to separate the liquid hydrocarbons from the groundwater by a semi-permeable membrane which is pervious to the liquid hydrocarbons. This method, however, is limited to such damage where an oily fluid tops the groundwater. The method cannot treat dissolved, dispersed or colloid pollutants.
A variety of patents relate to so-called circulation wells, such as DE 38 05 200 C1, DE 40 012 012C1 and DE 40 39 824C1, as well as commercial processes, such as the Hydroairlift process (Zueblin Inc., Stuttgart). According to this technology, a small (partial) vacuum (.DELTA.p.ltoreq.50 mbar) is applied onto the well head causing a vertical circulation of the polluted water in the aquifer with the treatment equipment in the center. When impressing the pressure drop, the water level will rise in the well approximately 1 cm per 1 mbar pressure drop. In general, in circulation wells the water is lifted by 22 to 50 cm above the normal water level. Once having been lifted in the well, the lifted water glides on the top of the water level and flows off the well. This water then will sink down at a certain distance from the well and is finally sucked in by the well at its lower end. The circulation is now complete. A prerequisite for the circulation, however, is a packer within the circulation well to separate the effluent from the take in. A packer (sealing) is also required for the annulus. The second prerequisite for a circulation well is a velocity of the groundwater of less than one foot/d; otherwise, the circulation will break down at least partially, discharging uncontrolled and only partially cleaned water into the aquifer. The pollutants are extracted by ambient air. The separation of the air and the pollutants is effected by a submerged stripping reactor. Official research of German regulating agencies could not find, however, clear evidence that this type of well works at all and that the claimed results can be achieved in real circumstances. See, for example, University of Karlsruhe on instruction of the Baden-Wuerttemberg State EPA: Documentation and Assessment of Selected Case Histories as to Halogenated VOCs (CVOCs) in Groundwater and the Remediation Thereof by Means of the UVB-Method; Karlsruhe 1990.
DE 40 06 435C2 discloses a method for an in-well stripping of dissolved volatile organic pollutants (VOCs) and to the flotation of liquid pollutants by gases other than ambient air, such as carbon dioxide and nitrogen. The gases act as an extraction agent to carry (strip) the pollutants out of the water as the gases move up and out of the well.
For electrochemical water treatment plants, the state of the art contemplates that organic substances can be decomposed at electrodes by means of anodic oxidation and cathodic reduction, such as disclosed in patents U.S. Pat. No. 4,988,427, U.S. Pat. No. 4,983,267, U.S. Pat. No. 4,383,901, AU-E21339B, DE 40 07 805 A1.
The underlying problem addressed by the present invention is to devise a low cost method and apparatus related to the in-well extraction or the in-well destruction of organic constituents in the water-bearing layers of the soil, irrespective of the viscosity of the pollutants and/or the precipitation of inorganic substances onto the well casing. It is a further object of the invention to provide a method and apparatus which will work effectively in aquifers having a low permeability (i.e. k.sub.f.gtoreq.10-5 m/s), and wells having a small output or situated in fizzurized rock where a circulation cannot be established. Another purpose of the present invention is that it does not change the water level in the ground.
According to the type of pollutant, the solution offered by the present invention is subdivided into the basic system dealing exclusively with volatile hydrocarbons (a CVS-type I well), and into an extended version. In the extended version, the CVS-type I well is upgraded so as to perform as an electrode. Then, by providing another electrode to serve as a counter electrode, appropriate reactions of anodic oxidation or cathodic reduction can be carried out within the well for the destruction of colloids and semi- and non-volatile pollutants, such as pesticides, PAH, TPH, MTBE, etc. and/or for the precipitation of inorganic substances (a CVS-type II).